Conventionally, in the field of optical communication or optical measurement, various light control devices such as a waveguide type optical modulator or a waveguide type optical switch where an optical waveguide or a controlling electrode is formed on a substrate having an electro-optical effect have become commercially available.
Most shapes of the light control device which is being used currently, as shown in FIG. 1(a), includes an optical waveguide 2 or a signal electrode 4 and a grounding electrode 5 which are formed on an electro-optical crystal substrate 1 having a thickness of about 0.5 to 1 mm. In addition, FIG. 1(a) illustrates an example of the optical modulator using a Z-axis cut substrate, and reference numeral 3 indicates a buffer layer such as SiO2 film.
Specifically, in the waveguide type optical modulator, a microwave signal is applied to the controlling electrode in order to control to modulate an optical wave propagated in the optical waveguide. Therefore, there is a need for achieving an impedance matching between a signal path, such as a coaxial cable which introduces microwaves into the optical modulator, and the controlling electrode in the optical modulator such that the microwave is efficiently propagated in the controlling electrode.
For this reason, as shown in FIG. 1(a), a shape of controlling electrode where a strip-shaped signal electrode 4 is interposed between grounding electrodes 5, that is, a coplanar type controlling electrode has been used.
However, in the case of the coplanar type controlling electrode, since an external electric field does not operate efficiently in a direction (corresponding to a vertical direction in the case of the Z-axis cut substrate shown in FIG. 1(a)) of high efficiency in the electro-optical effect of the substrate 1, a larger voltage is required in order to obtain a required optical modulation degree. Specifically, when a substrate made of LiNbO3 (hereinafter, referred to as “LN”) is used, and when an electrode length along the optical waveguide is 1 cm, a half-wavelength voltage of about 10 to 15 V is required.
As shown in FIG. 1(b), Patent Document 1 discloses a configuration that the optical waveguide is formed of a ridge type waveguide 20 and the grounding electrodes 5, 51, and 52 are disposed closer to the signal electrodes 4 and 41 in order to enhance an optical confinement of the optical waveguide and to more efficiently apply an electric field generated by the controlling electrode to the optical waveguide. With this configuration, it is possible to realize a reduction in driving voltage to some degree but it is essential to reduce the driving voltage further more in order to realize a high-speed modulation in a high-frequency band.
[Patent Document 1] U.S. Pat. No. 6,580,843
In addition, as shown in FIG. 1(c), Patent Document 2 discloses that the substrate is interposed between the controlling electrodes, and the electric field is applied in a direction (corresponding to a vertical direction in the case of the Z-axis cut substrate shown in FIG. 1(c)) of high efficiency in the electro-optical effect. Moreover, the optical modulator shown in FIG. 1(c) polarizes reversely the substrate having the electro-optical effect, and forms substrate regions 10 and 11 which are different from each other in a direction (a direction of arrow in FIG. 1(c)) of a spontaneous polarization, and the optical waveguide 2 is formed in each substrate region. When the electric field is applied to each optical waveguide by the common signal electrode 42 and the grounding electrode 53, it is possible to generate a phase variation in an opposite direction with respect to the optical wave propagated in each optical waveguide. Using this differential driving, it is possible to reduce the driving voltage further more.
[Patent Document 2] Japanese Patent Application No. 3638300
However, in the electrode structure shown in FIG. 1(c), a refraction index of the microwave becomes high, and thus it is difficult to realize a velocity matching between the optical wave which is propagated in the optical waveguide and the microwave which is a modulation signal. Moreover, since the impedance is reduced on the contrary, there is a drawback that it is difficult to achieve the impedance matching with the signal path of the microwave.
In addition, as the light control device using the polarization reversal, Patent Document 3 discloses a configuration that the signal electrode which configures the controlling electrode is branched into two or more in the middle of the path, and applies the same signal electric field to plural optical waveguides.
[Patent Document 3] Japanese Unexamined Patent Application Publication No. 2003-202530
In Patent Document 3, as shown in FIG. 2(a), a part of the Z-axis cut substrate is polarized reversely, a Mach-Zehnder optical waveguide (100, 101, and 102) is formed on the substrate, and further signal electrodes 103, 104 and 105 or grounding electrodes 106, 107 and 108 are disposed. The signal electrode is branched into two in the middle of the path to form two branched signal paths (signal electrodes 104 and 105).
In addition, FIG. 2(b) is a cross-sectional view taken on a dotted line A of FIG. 2(a). The branched waveguides 101 and 102 are disposed in different polarized regions (110, 111) in the Z-axis cut substrate, respectively.
As described above, when the signal path is branched into the multiple in the middle of the path, the signal paths are necessary to be set to have different impedances, for example, 50Ω for the signal path of the signal electrode 103, and 100Ω for the branched signal path of the branched signal electrodes 104 and 105 even though the signal paths are in the same light control device. Further, the branched signal path is required to be adjusted to have a very high impedance of 70Ω or more.
For this reason, it is very difficult to obtain the reduction in driving voltage or the velocity matching between the microwave and the optical wave while adjusting such impedances.
On the other hand, in the following Patent Documents 4 and 5, the optical waveguide and a modulation electrode are formed integrally in a very thin plate which has a thickness of 30 μm or less, and another substrate which has lower permittivity than the thin plate is bonded, so that an effective refraction index for the microwave is lowered and the velocity matching between the microwave and the optical wave is achieved.
[Patent Document 4] Japanese Unexamined Patent Application Publication No. 64-18121
[Patent Document 5] Japanese Unexamined Patent Application Publication No. 2003-215519
However, even though the controlling electrode is formed in the structure as illustrated in FIGS. 1(a) to 1(c) with respect to the optical modulator using such thin plate, the above-mentioned problems have still not been resolved fundamentally. When the substrate is interposed between the controlling electrodes shown in FIG. 1(c), the refraction index of the microwave tends to be decreased if the thickness of the substrate is thin, but it is difficult to realize the velocity matching between the optical wave and the microwave. Even though it depends on a width of the electrode, for example, when a thin plate made of LN is used, the effective refraction index is about 5 which does not come up to an optimal value of 2.14. On the other hand, the impedance tends to be decreased as the thickness of the substrate becomes thinner, which causes a mismatching in impedance to be large.